Place a large skillet over medium-high heat and cook the sausage, stirring frequently, until it is browned and crumbly, about 5 to 7 minutes. Drain off and discard the grease.

Add the celery to the cooked sausage and continue cooking, stirring occasionally, until the celery softens, approximately 10 minutes.

Transfer the sausage and celery mixture to a slow cooker. Add the zucchini, diced tomatoes, green bell peppers, chopped onion, salt, sugar, oregano, Italian seasoning, basil, and garlic powder to the slow cooker and stir to combine.

Cover and cook on the Low setting for 4 to 6 hours.

Serve the soup garnished with 1 tablespoon of grated Parmesan cheese per serving.